1. Field of the Invention
The invention relates to the field of ultrasonic scanners, and in particular to ultrasonic mechanical scanners for producing sector scans in an object to be scanned.
2. Prior art
Cross-Sectional Echography (CSE) is a commonly used technique for producing two dimensional images of cross-sectional slices of the human anatomy. So called real time or dynamic CSE is a technique whereby such images are produced sequentially at a frame rate sufficiently high to enable dynamic visualization of moving organs.
In CSE, the cross-sectional image is built up from a series of successive scan lines, each line being generated by the transmission of a short pulse of ultrasonic radiation into the object by an ultrasonic transducer, and subsequent detection and recording of the echoes reflected back to the transducer by the tissue structures within the object. The transmitted pulse is angularly confined by the use of a transducer aperture large compared to the wavelength of the ultrasound radiation to a rather narrow beam or pencil of radiation. The recording and display of the reflected echoes is analogous to that used in radar or sonar displays. In a linear scanner, the successive lines of a cross-sectional image are parallel to and placed one line spacing apart from one another. In a sector scanner, the successive lines are displaced angularly from one another and intersect at some common origin, which is the center of the scan. Displacement of the beams, and therefore scanning, is achieved either electronically, as is the case for phased array scanners, or by means of mechanical motions, as is the case for mechanical scanners.
While numerous sector scanners, both mechanical and electronic, are presently being marketed, these sector scanners all suffer from severe limitations. One of the existing scanners is the direct contact oscillating transducer mechanical scanner. In this scanner, exemplified by the devices described by J. Griffith et al, in "A Sector Scanner for Real Time Two Dimensional Echo Cardiography", Circulation, Volume XLIX, June 1974, and by Eggleton et al, in "Visualization of Cardiac Dynamics with Real Time Ultrasonic Scanner", Ultrasound in Medicine, D. White, ed., Plenum Press 1:385, 1975, a single transducer is oscillated about an axis nominally lying in the front plane and passing through the center of the transducer with a appropriate angle sensor being used to monitor the angular position of the transducer at any time. Contact with the patient is maintained by the use of a gel, and in operation the patient's tissues must conform to the movement of the transducer which is essentially rigid. Such a contact can result in poor acoustic coupling to the skin as well as an unpleasant vibrating sensation to the patient and resulting diagnostic difficulties. In addition, the direct contact mechanical sector scanners are limited in their useful scanning angle by the problems of the moving contact and physical angulation of the transducer away from the skin, in most cases to values of 30 to 45 degrees. A further limitation common to all oscillating mechanical sector scanners is that their angular rate of sweep is not uniform, since the transducer or mirror system must reverse direction at the end of each sweep in each direction. As a result the line density is greatest at the edges of the sector, where it is usually least desirable, and is lowest at the center of the sector, i.e., the center of the region of interest. Concomitant with this limitation is the fact that the alternate direction of sweep means that an area at the end of a sweep is interrogated twice in a very short interval, as the scan crosses it in opposite directions, and is not interrogated again until nearly the duration of two frames. In addition, only the mid-point of the scan is interrogated at a constant frame rate. Finally, the direct contact mechanical (and phased array) scanners cannot properly visualize the tissues near the patient's skin because of the large acoustic pressure field non-uniformities occuring in the Fresnel zone extending a distance D=(d.sup.2 /4.lambda.) (where d is the transducer diameter and .lambda. is the mean acoustic wavelength) in front of the transducer. Typically, such a region extends three to four centimeters and thus can include portions of the body which are of diagnostic interest.
Another type of existing scanner is the oscillating transducer water bath scanner. In this scanner, exemplified by the device described by Nakashika et al, in "Real Time Cardiactomograph with Handy Water Immersed Sector Scan System", Proceedings 1st Meeting of World Federation of Ultrasound in Medicine and Biology, San Francisco, Calif., August, 1976, the oscillating transducer is set inside a liquid filled head having a thin membrane or window which is substantially acoustically transparent and which is in contact with the patient. This provides a stationary contact with the patient, with the acoustical coupling to the patient being provided by the water or other suitable liquid (e.g. silicone oil) and by the window. While the moving contact problem described above is alleviated, the other stated drawbacks of oscillating transducer scanners remain. Furthermore, the center of the sector scan in such devices is located within the scanner head, behind the scanner/patient skin interface, which frequently leads to artifacts in the recorded images due to the interfering effects of anatomical structures, such as ribs, when examinations are attempted through the relatively small intercostal spaces, generally referred to as the rib interference problem. The mechanical oscillation of the transducer in the surrounding liquid also generates acoustical noise and reduces the sensitivity of such scanners.
A further type of existing scanner is the water bath mechanical scanner with oscillating mirror, exemplified by the 150 S-4 Real Time Ultrasound Scanner, sold by the Xerox Corporation. In this scanner, an oscillating flat mirror is positioned in front of a stationary transducer and reflects ultrasonic pulses from the latter, the sector angle thus scanned being twice the angular excursion of the mirror. The scanner head makes a stationary contact with the patient, the transducer and mirror are immersed in a suitable liquid, and an acoustically transparent window is used, thus permitting the propagation of the acoustic pulses within the instrument and acoustic coupling to the patient. While this device has certain advantages over the previously described water bath scanner, the basic limitations of this device are the same as those of such water bath scanners with the further disadvantage that the sector scan center is placed still further behind the skin/window interface because of the size of the mirror required to intercept the transducer beam over the whole range of scan angles.
Another class of existing scanners is the rotating multiple transducer water bath scanner. This scanner is exemplified in the device described by Barber et al, in "Duplex Scanner II: For Simultaneous Imaging of Artery Tissues and Flow," IEEE 1974 Ultrasonics Symposium Proceedings", and in a device marketed under the name Eko-Sector I (TM) and offered commercially by the Smith-Kline Corporation. In these devices, four transducers are mounted on the rim of a rotating wheel or cylinder, and the wheel is immersed in a suitable liquid filling the scanning head, with a suitable coupling window providing the stationary contact interface to the patient. The Smith-Kline device has a cylindrical exit window placed very close to the rim of the rotating cylinder while the Barber device has a flat window several centimeters away from the transducers. The transducers are electrically connected to the acoustic pulsing and receiving electronics by means of a suitable commuting contact arrangement and are switched in succession as the wheel rotates, so that only the transducer sweeping the desired sector (i.e., facing the window is active. While the rotating head configuration provides the desired regular angular line spacing and a non-reciprocating scan and avoids vibration and the generation of noise in the scanner head since the moving element is undergoing a continuous rotational motion, it suffers from the typical limitations relating to transmitting and receiving electrical signals to and from moving transducers and in particular, is sensitive to electrical contact noise which is generally quite significant at the very low signal levels corresponding to pulse echo returns. In addition, since four or more transducers are used in sequence, these must be carefully matched and positioned in order to avoid artifactual changes in the successive images generated by the transducers. Furthermore, these rotating transducer scanners retain the limitation of the oscillating single transducer water bath scanners that the sector scan center is located at the center of the rotating wheel and thus well behind the scanner patient interface, and therefore are subject to the rib interference problems that arise from such scanning geometry.
Existing phased array scanners are exemplified in articles by M. G. Maginness et al, "State-of-the-art in Two-Dimensional Ultrasonic Transducer Array Technology", Medical Physics, Vol. 3, No. 5, Sept./Oct. 1976, Von Ramm et al, "Cardio-Vascular Diagnosis in the Real Time Ultrasound Imaging", Acoustical Holography, Vol. 6, 1975, and J. Kisslo et al, "Dynamic Cardiac Imaging Using a Phased-Array Transducer System", published by Duke University, Durham, N.C. In such scanners a large (16-60 element) linear array of small transducers is used, with a variable time (phase) delay inserted between elements of the array both in the transmission and reception of the ultrasound signal, resulting in a transmitted beam and a receiving beam or sensitivity pattern whose direction is determined by the magnitude of the inter-element time delay. In sector scanning using phased array scanners, such scanning is achieved without any mechanical motion of the transducer array which remains in stationary contact with the patient's skin. Such phased array scanners have, however, several practical limitations. One such limitation resides in the relative complexity of the multi-element transducer array and especially of the trasmit/receive electronics necessary to achieve electronic beam steering, resulting in a relatively high cost of phased array scanners. In addition, the ultrasonic beam quality in phased array scanners, in terms of lateral resolution and side lobe levels and the possible occurance of grating lobes, is poor compared to that of single transducer scanners, particularly for beam direction angles greater than 30 degrees away from the normal to the transducer.
In addition to the various limitations of the scanners described above, all existing sector and linear scanners, both mechanical as well as phased array, are limited to operating at a single frequency so that a different scanning head must be installed for each frequency and can generate the sector scan with only one beam at a time. Moreover, the image producing capabilities of many existing scanners are restricted by the existence of "echo" artifacts which degrade the quality of and complicate the interpretation of the reflected signals from the object being visualized, such echo artifacts being caused by ultrasound energy being received by a detector which energy is not directly reflected from the body or target under examination.
Accordingly, it is a general object of the present invention to provide an improved ultrasonic sector scanner.
It is another object of the present invention to provide a sector scanner which has a sector scan center of focus which can be located at the scanner/skin interface or beyond to minimize interference problems.
It is a further oject of the present invention to provide a sector scanner which has stationary transducers and requires no sliding or intermittent electrical contacts between the transducers and the transmit/receive electronics.
It is another object of the present invention to provide a sector scanner which has a stationary contact with the object being scanned and is free of vibration problems.
It is still another object of the present invention to provide a sector scanner which has a uniform line density and sampling rate at all angles, and high quality radiating and receiving beam patterns.
It is another object of the present invention to provide a sector scanner which is free of echo artifacts.
It is a further object of the present invention to provide a sector scanner in which no part of the body of diagnostic interest lies in the Fresnel zone of large variations of acoustic intensity.
It is still a further object of the present invention to provide a sector scanner with increased line density and/or frame rate.
It is another object of the present invention to provide a sector scanner which utilizes only a single scanner head yet which can operate at two or more frequencies.